drivers: i2s: fix block size for DMA interface
drivers: i2s: fix block size for DMA interface

`block_count` in `dma_cfg` is described as how many bytes to be
transfered in dma.h. So it should be 2 since the source data size
and dest data size are all 16 bits in this application. And all
block size should represent just bytes.

Signed-off-by: Song Qiang <c598b3ff423ba743d91ae7caa6428138a985a8a3@gmail.com>

nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr